1. Field of the Invention
The present invention relates to a thermal transfer sheet which is used in a recording system for thermally transferring a coloring material with heat-application means such as a thermal head.
2. Description of the Related Art
With the development of office automation, a thermal transfer recording system is used in various terminal equipments such as a facsimile machine or a printing machine. This recording system is advantageous over other recording systems such as electrophotographic type or ink jet type ones because of easy maintenance of the machine, easy operability, low costs of the machine and consumables.
Since the thermal transfer sheet is heated with a thermal head at high temperature in the thermal transfer recording system, if a base film of the thermal transfer sheet has insufficient heat resistance, it is fusion-bonded to the thermal head, which results in generation of sticking noise or deposition of thermal head tailings. When the fusion bonding becomes severe, running of the thermal head is impossible so that no recording is possible.
To solve this problem, Japanese Patent Kokai Publication No. 7467/1980 proposes the supply of a heat resistant resin layer such as a silicone resin or an epoxy resin on the base film, and Japanese Patent Kokai Publication No. 129789/1982 proposes the supply of a resin layer containing a surfactant which is in a solid or semisolid state at room temperature on the base film. However, these measures require a large amount of energy for heat-curing the resin. In addition, the base film suffers from deformation wrinkles caused by heat, so that the coloring material layer is irregularly coated on the base film and therefore printing quality is deteriorated. When the surfactant is added to the resin layer, it will adhere to the thermal head to cause printing slips.
With the increase of the energy applied to the thermal head due to recent high speed printing, the thermal transfer sheet receives large load. Then, it is difficult to improve the runnability of the thermal head by the methods disclosed in the above Japanese Patent Kokai Publications.
As further methods for solving the above problem, Japanese Patent Kokai Publication No. 27289/1986 proposes the supply of a heat resistant layer of a UV-curable resin such as polyester acrylate on the base film and Japanese Patent Kokai Publication No. 207679/1987 proposes the supply of a cured layer of a silicone having radically polymerizable double bonds.
While the base film having such layer is excellent in heat resistance, it has an insufficient slipping property, so that the runnability of the thermal head may not be improved, or the uncured silicone migrates on a surface of the base film on which the coloring material layer will be formed. Thereby, the coloring material layer is irregularly coated. In addition, the uncured silicone migrates in the coloring material layer so that the normal printing is interfered.
To reduce recording cost in the thermal transfer recording system, a repeated use type thermal transfer sheet is being developed. Therefore, a heat resistant layer is required, which does not deteriorate the runnability of the thermal head even when it is repeatedly heated by the thermal head and prevents deposition of the thermal head tailings.